1. Field of the Invention
The present invention relates to a lithographic projection apparatus and a method of manufacturing a device.
2. Brief Description of Related Art
The term “patterning device” or “patterning structure” as here employed should be broadly interpreted as referring to a device or structure that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below).
An example of such patterning device includes a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of such patterning device includes a programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing a piezoelectric actuation device. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The matrix addressing can be performed using a suitable electronic device. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Patents U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference thereto. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable.
Another example of such patterning device includes a programmable LCD array. An example of such a construction is given in U.S. Patent U.S. Pat. No. 5,229,872, which is incorporated herein by reference thereto. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are provided, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference thereto.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference thereto.
In the above apparatus, the mask and substrate may each be securely held (“clamped”) so that it can be accurately positioned in the X, Y and Z directions and in rotational orientation about the X, Y and Z axes (referred to as the Rx, Ry and Rz directions). The Z direction is defined as being the direction substantially perpendicular to the plane of the mask or substrate in question (which defines the XY plane). The mask and substrate can be subjected to very large accelerations in their plane, particularly in a step-and-scan apparatus. Accurate positioning of the mask or substrate also requires relatively high stiffness in the Z direction. The clamping arrangement is sufficiently secure to withstand such accelerations and also to provide the necessary stiffness.
Previous clamping arrangements, such as a rigid vacuum clamp, have the problem that deformation of the mask can be caused. This can be as a result of either or both of the mask and the vacuum clamp not being perfectly flat or because of contaminant particles being trapped between the mask and the clamp. The deformation of the mask or substrate leads to distortion of the exposed image which can lead to overlay errors.
A previous attempt to reduce the problem of deformation is to use a membrane which is compliant in the Z direction, for example as disclosed in U.S. Pat. No. 5,532,903, to support the mask. However, this still suffers from the problem of contaminant particles between the membrane and mask and also their lack of rigidity and stiffness.
In order to alleviate the sensitivity to contamination between the mask or substrate and the supporting structure or table, referred to as a chuck, a pimple plate has previously been used between the mask and chuck. The tips of the pimples define the plane on which the mask or substrate is supported, and the spaces between the pimples can receive the contaminants without deforming the plane of the mask or substrate. However, the use of a pimple plate has the problem that three surfaces need to be accurate, namely the top and bottom of the pimple plate and the surface of the chuck on which the bottom of the pimple plate is held. An alternative is to put pimples directly on the chuck, but these can be damaged by cleaning, in which case the whole chuck is damaged, which is costly to replace.